The performance of free Candida antarctica lipase B (CALB) in a mixed solvent system for enantioselective esterification of (R)-ketoprofen, leaving the target product (S)-ketoprofen in unreacted form was investigated. The effects of enzyme loading, substrate concentration, alcohol to acid molar ratio, type of solvents and reaction temperature were studied. The optimum reaction conditions obtained were 200 mg/ml of CALB and racemic ketoprofen concentration of 30 mM, with 20% (v/v) 1,2-dichloropropane-80% (v/v) n-hexane as solvent, alcohol to acid molar ratio of 1: 1 at 40 degrees C. The conversion of (R)-ketoprofen, X-R obtained was 81% with enantioselectivity (E) value of 10, enantiomeric excess of product (ee(p)) of 66.4% and enantiomeric excess of substrate (ee(s)) of 59.4% for the reaction time of 24 h. (c) 2006 Elsevier Inc. All rights reserved.